A septimal neutral seventhplay (help·info) has a ratio of 64:35 or about 1045 cents.
The just undecimal neutral seventh has a ratio of 11:6 between the frequencies of the two tones, or about 1049 cents play (help·info). Alternately, 13:7 or about 1071.7 cents.
A tridecimal neutral seventhplay (help·info) has a ratio of 24:13 between the frequencies of the two tones, or about 1061 cents. This is the largest neutral seventh, and occurs infrequently in music, as little music utilizes the 13th harmonic.
An equal-tempered neutral seventhplay (help·info) is characterized by a difference in 1050 cents between the two tones, a hair larger than the 11:6 ratio, and exactly half of an equal-tempered major thirteenth (octave plus major sixth).
These intervals are all within about 12 cents and are difficult for most people to distinguish. Neutral sevenths are roughly a quarter tone sharp from 12 equal temperament minor sevenths and a quarter tone flat from 12-ET major sevenths. In just intonation, as well as in tunings such as 31-ET, 41-ET, or 72-ET, which more closely approximate just intonation, the intervals are closer together.
A neutral seventh can be formed by stacking a neutral third together with a perfect fifth. Based on its positioning in the harmonic series, the undecimal neutral third implies a root one perfect fifth below the lower of the two notes.